Being billed for contract that expired

My Telkom mobile contract expired at the end of September 2017. I have send the cancellation form in August 2017 to ensure that the contract does not continue on a month to month basis. In October 2017 I received confirmarion that the cancellation form was received. I am still receiving a debit order request every month at different dates in the month for a contract that is no longer valid. At this stage I have to go into the branch to reverse this every month as they are not using the standard instruction as I have blocked this. Please can someone assist in getting these debit orders to be stopped.